1. Technical Field
This application relates generally to fluid processing. More particularly, this application relates to chemical analysis of fluid samples within a wellbore environment.
2. Background Information
Chemical analysis is a critical step in the evaluation of the hydrocarbon reserves. The fluid/gas composition has a large impact on the economic value of the reservoir. Furthermore, the fluid/gas composition determines the well completion and production strategies. Traditionally, samples are taken in the field, shipped to a laboratory, often reconstituted to reservoir conditions and then analyzed.
Many components have to be analyzed downhole due to changes as a result of the sampling. For example, the pH of a water sample can change due to the outgassing of carbon dioxide (CO2) or hydrogen sulfide (H2S). Hydrogen sulfide in gas or oil can be scavenged by metal parts or the sample bottle and barium in water can even precipitate as barium sulfate before the sample is taken.
Spectroscopic techniques are able to determine some components in the oil/gas without any preparation. An example of this is the compositional analysis as performed by an analyzer, such as the Compositional Fluid Analyzer (CFA) module of the Modular Formation Dynamics Tester (MDT), a tool suite commercially available from Schlumberger Technology Corporation, Sugar Land, Tex. However, the number of components that can be determined directly by spectroscopic techniques is limited. Adding a color agent (dye) to the solution to determine one component of the fluid has been proven to be a successful method for the determination of pH (e.g., using a Live Fluid Analyzer, LFA-pH module of the MDT).
Within certain limits, the dye concentration is generally of little or no importance in the case of a pH measurement. However, pH measurements are the exception and most other measurements require a known mixing ratio between reagent and sample. An example is a newly developed method to determine hydrogen sulfide concentration in oil, gas or water by a colorimetric reaction with a metal ion.
Titration is a common method to determine the concentration of a target component in solution. In a titration one reagent is slowly added to a sample solution of the target component (or vice versa) until a sudden event (e.g., color change, precipitation, or other observable change) takes place. The slow addition of one component (reagent) to a solution of another component (target) equates to a slow variation of the mixing ratio of the two components. However, in order to determine the concentration of the target component, the final mixing ratio has to be known. An example relates to determining alkalinity of a solution (sample). The sample is slowly titrated with acid in the presence of a pH sensitive dye, until a color change takes place due to the pH sensitive dye responding to a pH of the titrated sample.
A common approach in chemical analysis is the use of flow injection analysis (FIA). FIA is a helpful technique, particularly for situations in which a chemical sensor may not be very stable, only small amounts are available, or when a reaction product has to be measured in-situ. The FIA technique can be used to compare a mixture's response to an injection of reagent with a baseline response. FIA measurements can compensate for drift in a detector or in case of a colorimetric reaction, for the background coloration of the reagent.
Chemical analysis, particularly in the evaluation of the hydrocarbon reserves, will very likely use more and more chemicals that may not be “environmental friendly.” At least one such example relates to analysis of a sample to detect the presence of hydrogen sulfide in oil and gas, in which a reaction with metal ions is suggested as a suitable sensing technique. Suitable metals for use in such situations can include cadmium which is known carcinogenic. Thus, collecting the waste of such chemical reactions would be desirable, as an example of good citizenship. Furthermore, some environmentally sensitive areas (e.g., Alaska) require that no chemicals be left behind during testing and production of an oil well.